Ok, so I'm confused again. What is the marble and what is the trolley? And why is the double-layer set up to "point" in one direction instead of the opposite direction?

And most particularly, are the marbles positively charged or negatively charged? In either case, how come the exhaust is neutrally charged?

It doesn't make any sense, to me.

My understanding is the a double layer will accelerate ligher electrons in one direction, and heavier ions in another. Equal numbers of oppositely charged ions are accelerated in opposite direction.

But because ions are at least 1836 times heavier than electrons (if the ions were protons), and because of equal and opposite reactions, as the heavier ions are accelerated in one direction, the spacecraft engine reacts by moving in the opposite direction.

So the analogy with a marble has the spacecraft's electric field being the counterpart of gravity. And as we know, electromagnetic forces are 10^39 times more powerful than gravity. So while an ion is "falling" through the double layer, a constant force is applied push, and the engine reacts in the opposite direction.

And this is why Hannes Alfvén made such a big deal about cosmic double layers [Ref].

Regards,
Ian Tresman

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Ian Tresman
plasma-universe.com